Internal combustion engines can utilize turbochargers or superchargers to obtain increased intake air pressure to enable increased engine performance. Turbochargers typically include an intake air compressor rotationally coupled with an exhaust gas turbine, where the turbine provides thrust to the compressor by extracting energy from the exhaust flow. Some of turbochargers may include a motor to assist the compressor develop sufficient thrust when the turbine is unable to extract enough energy from the exhaust flow or where the exhaust flow has insufficient energy to power the compressor.
The inventors of the present application have recognized that one disadvantage of the above turbocharger is that the compressor and turbine are rotationally coupled, which can limit the operating state at which the compressor and turbine can be operated. For example, compressor surge can be reduced by increasing an amount of exhaust flow bypassing the turbine, which can be achieved by increasing an opening of a wastegate arranged in a bypass passage of the turbine. By reducing the amount of exhaust energy that is extracted by the turbine, the speed and resulting boost pressure provided by the compressor can be reduced due to the rotational coupling, thereby reducing or eliminating compressor surge. However, by increasing the bypass flow of exhaust gases, the efficiency of the engine system may also be reduced since a greater amount of exhaust energy is transferred to ambient by way of the wastegate without being extracted by the turbine.
The inventors have provided various approaches, disclosed herein, that may address at least some of the above issues by a vehicle propulsion system that comprises: an internal combustion engine including at least one cylinder; an intake air compressor communicating with the cylinder via an intake valve; an electric machine rotationally coupled to a shaft of the compressor; an exhaust gas turbine communicating with the cylinder via an exhaust valve; and a control system configured to operate the compressor at a different speed than the turbine, at least under an operating condition, and to adjust the electric machine responsive to conditions of compressor surge to reduce the surge while continuing to extract energy from an exhaust flow of the engine via the exhaust gas turbine. Note that the turbine wastegate may still be used with this approach to control turbine operation, however, by adjusting the electric machine rotationally coupled with the compressor to adjust compressor speed and/or torque, surge may be reduced or avoided, enabling a reduction in the amount of exhaust flow that is diverted through the turbine bypass. In some examples, the wastegate and turbine bypass passage may be entirely eliminated, thereby reducing the cost and complexity of the engine system. By reducing the turbine bypass, at least under some conditions, the turbine can be operated to generate electrical energy via a second electric machine rotationally coupled with the turbine, which in turn can be used power the electric machine of the compressor, or other loads.
As another approach described by U.S. Pat. No. 6,647,724, some of the potential drawbacks of a rotationally coupled compressor and turbine can be addressed by providing a turbocharger that includes an electric compressor to boost intake charge pressure supplied to an internal combustion engine and an electric turbine to generate electrical power from exhaust received from the engine. By operating the electric compressor mechanically independently of the turbine, turbocharger lag may be reduced during conditions where a more rapid increase in boost pressure is requested. However, the inventors of the present disclosure have also recognized that adjustment to the compressor operating state independent of the operating state of the turbine can impact the amount of exhaust gas residuals that are retained in the engine cylinders for a given operating condition. If an insufficient amount or concentration of residuals are retained by the engine or if residual retention is too great for the given operating conditions, then the efficiency of the system may be reduced, misfire may occur, and/or noise, vibration, and harshness (NVH) of the engine may be increased.
The inventors have provided another approach, disclosed herein, that addresses at least some of the above issues of residual retention, which includes as one example, a vehicle propulsion system, comprising: an internal combustion engine including at least one cylinder; an intake valve operable to selectively admit at least intake air to the cylinder; an exhaust valve operable to selectively exhaust products of combustion from the cylinder; an intake air compressor communicating with the cylinder via the intake valve; an exhaust gas turbine communicating with the cylinder via the exhaust valve; and a control system configured to operate the compressor at a different speed than the turbine, at least under an operating condition, and to adjust an amount of opening overlap between the intake valve and the exhaust valve in response to a rotational speed of the compressor. The control system may also adjust the amount of opening overlap in response to the rotational speed of the turbine or a speed difference between the turbine and compressor. In this way, valve timing may be adjusted to retain a prescribed amount of residuals in the engine cylinders even when the operating state of the compressor and turbine are adjusted independently of each other.
The inventors of the present disclosure have recognized further issues with regards to the previous approaches. For example, some engines may operate in one of a plurality of different combustion modes, which can each utilize different levels of boost as well as different levels of exhaust gas residual production and retention. If boost is simply increased by adjusting the compressor, the amount and/or concentration of residuals retained by the cylinders may also change, thereby potentially impacting the performance of the engine depending on the combustion mode presently utilized. For example, two stroke combustion modes can have higher residual retention rates than four stroke combustion modes, since the valve overlap timings may be shorter in the two stroke mode. As another example, spark ignition combustion modes may utilize a lower concentration of residuals from a previous combustion cycle than a homogeneous charge compression ignition mode where autoignition is utilized to ignite the fuel and air mixture.
The inventors have provided yet another approach, disclosed herein, that addresses at least some of the above issues relating to residual retention, which includes as one example, a method of operating an engine, comprising: during a first operating condition, operating the engine in a first combustion mode while adjusting the compressor to provide a higher level of boost to the engine and operating the turbine at a first speed difference relative to the compressor; and during a second operating condition, operating the engine in a second combustion mode while adjusting the compressor to provide a lower level of boost to the engine and operating the turbine at a second speed difference relative to the compressor. In this way, a suitable level of boost can be provided to the engine based on the combustion mode presently being used, while also retaining an amount of exhaust gas residuals in the engine cylinders that is appropriate for the selected combustion mode.
The inventors of the present disclosure have recognized still further issues associated with an engine utilizing a rotationally uncoupled compressor and turbine pair. For example, during warm-up of the engine, if the turbine is operated to extract exhaust gas energy from the exhaust stream, then the exhaust treatment devices may take longer to reach their prescribed operating temperature. However, during these conditions, additional boost pressure may be requested. Thus, as one example, a method of operating a vehicle propulsion system including an internal combustion engine having an intake air compressor and an exhaust system including an exhaust turbine having a turbine generator and an exhaust treatment device arranged downstream of the turbine is provided. The method comprises: during a lower temperature condition of the exhaust system, operating the turbine generator to convert a lesser amount of exhaust gas energy produced by the engine to electrical energy and operating the compressor motor at a first speed difference relative to a speed of the turbine generator; and during a higher temperature condition of the exhaust system, operating the turbine generator to convert a greater amount of exhaust gas energy to electrical energy and operating the compressor motor at a second speed difference relative to a speed of the turbine generator less than the first speed difference. In this way, the amount of exhaust gas energy removed from the exhaust flow can be adjusted in response to the temperature of the exhaust treatment device to promote rapid heating of the exhaust system, while also providing sufficient boost pressure to the engine by way of the compressor.
The inventors of the present disclosure have recognized still further issues. For example, during some operating conditions, a reduced level of engine boost may be prescribed in order to meet the torque request of the vehicle operator, while during other operating conditions, an increased level of engine boost may be prescribed. The inventors have recognized that a reduction in engine boost, facilitated by reducing compressor speed and/or torque, need not necessarily be accompanied by a corresponding reduction in turbine torque or speed when the turbine is rotationally uncoupled from the compressor. For example, during an idle state of the engine, the turbine may be operated to extract energy from the exhaust flow even while the compressor is not operated or when the compressor is operated to a lesser extent. However, the compressor can serve as an obstruction to the intake system during conditions where additional boost is not prescribed for the given engine operating state. Thus, engine efficiency can be reduced during compressor inactivity since the engine entrains air through the compressor.
As such, the inventors have provided an approach that addresses this issue, as one example, by a method of operating an internal combustion engine including a first intake air compressor rotationally coupled with a first electric machine and an exhaust turbine rotationally coupled with a second electric machine, the method comprising: in response to a first operating state of the engine: increasing a flow of intake air through a bypass passage of the compressor relative to a flow of intake air through the compressor by opening a compressor bypass valve; and generating a first amount electrical energy with the second electric machine by extracting exhaust gas energy flowing from the engine via the turbine; and in response to a second operating state of the engine: decreasing the flow of intake air through the bypass passage of the compressor relative to the flow of intake air through the compressor by closing the compressor bypass valve; supplying electrical energy to the first electric machine to increase the rotational speed of the compressor; generating a second amount of electrical energy greater than the first amount with the second electric machine by extracting exhaust gas energy from the engine via the turbine; and adjusting the speed of the turbine relative to the compressor to vary the second amount of electrical energy generated by the second electric machine. In this way, engine efficiency may be increased during conditions where a lower amount of boost is requested or where the compressor is inactive.